I believe there are many gods, not one, and always in conflict. Well, if there are gods who created and control whatís going on here, you can tell a lot about them by what is going on. I assume they are colonists. And so anything you can see here you can infer is the act or the will of one of those gods. All you have to do is look around. Youíll see what theyíre like. Itís like looking in someoneís yard.
Ė W.S. Burroughs

Dateline: 1950 A.D. The physicist Enrico Fermi is having lunch with a few colleagues when the subject of interstellar travel comes up. There are countless potential living systems in our own galaxy, Fermi reasons. If even a small fraction of these produced technological civilisations capable of launching manned expeditions, our own humble planet should have been overrun with extraterrestrial conquistadores millions of years ago.
ďDonít you ever wonder where everybody is?Ē Fermi asked.

In other words, if the galaxy is filled with little green men in luminous flying saucers who travel from star to star colonising other worlds, then why donít we have any physical evidence of their existence Ė not even so much as a piece of broken zipper from one of their spacesuits?

This question (known as the Fermi paradox) has spawned a vigorous scientific and philosophical debate whose possible answers can be grouped into three broad categories.

1. Extraterrestrial civilisations do not exist.

2. Extraterrestrial civilisations exist but havenít colonised the Earth, either because they canít or because they donít want to.

3. Extraterrestrial civilisations exist and have colonised the Earth without our knowledge.

As we shall see, the implications for all of these positions are unsettling in the extreme Ė not so much for what they say about extraterrestrial life, but for what they might say about us.